Cu(hfac)2 Complexes with Acyclic Nitroxide Prone to Single-Crystal to Single-Crystal Transformation and Showing Mechanical Activity

The heterospin solid phases of the chain polymer [Cu­(hfac)2LEt]∞ and bicyclic molecule [Cu­(hfac)2LEt]2-I (LR = pyrazolyl-substituted tert-butylnitroxide; 1-R-5-(tert-butyl-oxylamino)­pyrazole, R = Et, Pr) were found to undergo spontaneous transformation into the bicyclic molecule [Cu­(hfac)2LEt]2-II. The single-crystal to single-crystal (SC–SC) transformation of [Cu­(hfac)2LEt]2-I into [Cu­(hfac)2LEt]2-II was recorded by X-ray diffraction analysis of the crystal as a function of time. At 255–277 K, the [Cu­(hfac)2LEt]2-I → [Cu­(hfac)2LEt]2-II SC–SC transformation proceeded for 12–18 h. The [Cu­(hfac)2LEt]∞ → [Cu­(hfac)2LEt]2-II SC–SC phase transformation was accompanied by a change in the crystal shape, spontaneous mechanical displacements of crystals, and a change in color from orange to dark green. This process started, to a certain extent, already in the crystals lying under the layer of the mother solution. After the crystals were separated from the solution, the SC–SC transformation [Cu­(hfac)2LEt]∞ → [Cu­(hfac)2LEt]2-II occurred completely within 4 h at room temperature. Under normal conditions, [Cu­(hfac)2LPr]2-I also undergoes transformation into [Cu­(hfac)2LPr]2-II. At the macro level, the transformation [Cu­(hfac)2LPr]2-I → [Cu­(hfac)2LPr]2-II is accompanied by spontaneous fragmentation of crystals, visualized as a scatter of small particles of the formed phase in different directions. The reverse transformation [Cu­(hfac)2LPr]2-II → [Cu­(hfac)2LPr]2-I occurs when [Cu­(hfac)2LPr]2-II is cooled below 225 K. When [Cu­(hfac)2LPr]2-II was heated above 300 K, the irreversible SC–SC phase transformation [Cu­(hfac)2LPr]2-II → [Cu­(hfac)2LPr]∞ was observed, which caused a pronounced change in the color of the crystals from dark green to orange. Heat treatment of the [Cu­(hfac)2LPr]∞ single crystal at 303 K on a diffractometer for 1 day or more caused partial melting of the starting crystal, disappearance of X-ray diffraction reflections from the sample under study, and appearance of reflections corresponding to the formation of the new polymer complex [Cu­(hfac)2L*Pr]∞, where L*Pr is the product of transformation of the radical including the oxidation of LPr and migration of the nitroxide O atom to the heterocycle, leading to the formation of 5-(tert-butylimino)-1-propyl-1,5-dihydro-4H-pyrazol-4-one (L*Pr). The results of the X-ray diffraction study of the phase transformations completely agreed with the data of magnetochemical measurements for the com